1. Field of the Invention
The present invention relates to an external force measuring apparatus which receives signals output from an external force detecting means constituted by, for example, an accelerator sensor and an angular velocity sensor, eliminates components such as random noises and drift noises contained in the signals and thereafter integrates and outputs the signals.
2. Description of the Related Art
A conventional external force measuring apparatus for detecting a rotational angle generally detects a rotational angle of an object by integrating angular velocity signals of the object. Since the external force detecting means employs an angular velocity sensor, low-frequency noises such as random noises and drift noises are superimposed on the detection signals issued from the angular velocity sensor. Hence, there are various methods whereby to extract only signal components and integrate them in order to remove such noises.
For example, Japanese Laid-open Patent Publication No. 4-269622 discloses a tuning fork type vibration gyroscope for detecting a displacement angle. The vibration gyroscope comprises a detector and two drift detectors having the same configuration as that of the detector without a tuning fork. Drifts of the detector can be eliminated by subtracting the mean value of output signals of the two drift detectors from the output signal of the detector since drift components are superimposed on the output signals of the two drift detectors in the same way as that of the detector.
In recent years, a method for canceling random noises or drift noises by combining an angular velocity sensor with other angle detecting elements has been proposed, for example, in Japanese Laid-open Patent Publication No. 5-71964. In this publication, an angular velocity sensor and a geomagnetism sensor are employed to accurately calculate an azimuth. In this method, the angular signal obtained by integrating the signal of the angular velocity sensor is subtracted from the angular signal obtained from the geomagnetism sensor thereby to remove disturbance components contained in the signal issued from the geomagnetism sensor so as to determine accurate azimuth.
Further, Japanese Laid-open Patent Publication No. 3-188317 has proposed a method for calculating of angles with high accuracy by estimating drifts which fluctuate in probability mode using a nonlinear filter such as a Kalman filter. In this publication, the outputs from the angular velocity sensor and the geomagnetism sensor are read in and the current angle is calculated from these values and a past estimated angle. At this time, the detection signal from the angular velocity sensor is determined with high reliability by including an offset deviation. The Kalman filter theory is employed for this purpose. According to the Kalman filter theory, noise components can be effectively removed by performing arithmetic operation on the outputs of a three-axis angular velocity sensor by using the Kalman filter.
Among the conventional arts described above, however, the one in which a plurality of sensors are combined has a shortcoming in that the use of a plurality of sensors inevitably results in a complicated configuration of the apparatus, adding to the cost.
On the other hand, estimating a true detection signal from a detection signal by the Kalman filter requires an enormous amount of calculation; hence, it is necessary to employ a high-performance digital signal processor (DSP), a Reduced Instruction Set Computer (RISC) processor, or the like to process the detection signals from the sensors in real time, presenting a disadvantage of higher cost.
As an apparatus which overcomes the shortcomings described above, there has been known an offset drift correcting apparatus as disclosed in Japanese Laid-open Patent Publication No. 7-324941.
The offset drift correcting apparatus according to the art is constituted by: an angular velocity sensor for detecting angular velocity of a mobile unit; a turn determining means for determining whether the mobile unit is turning; and an adaptive filter which estimates the offset of the angular velocity sensor by using an average angular velocity obtained by smoothing corrective angular velocities and makes corrections if it is determined by the turn determining means that the mobile unit is not turning.
In the offset drift correcting apparatus, the difference between the output of the angular velocity sensor and the output from the adaptive filter estimated at that time provides the error amount of the estimated value. The value is averaged and is updated using the adaptive filter only when it is determined that the object is not turning. Thus, after a sufficient time elapses, the offset level of the angular velocity sensor settles to the output from the adaptive filter. The corrected angular velocity obtained by subtracting the estimated offset level from the angular velocity sensor is therefore processed as an accurate angular velocity so as to prevent the detection signal from being subjected to temperature drift, allowing drift components to be canceled.
In the offset drift correcting apparatus in accordance with the prior art described above, the output from the adaptive filter is always set at a value lower than the detection signals output from the angular velocity sensor thereby to enable the removal of drift components from detection signals.
However, the conventional art, poses a new problem in that, even if the turn determining means determines that the mobile unit is not turning, the difference between the detection signal and the output of the adaptive filter cannot be offset to zero, making it impossible to achieve reliable offset adjustment.
Accordingly, the present invention has been made in view of the problems with the prior art described above, and it is an object thereof to provide an external force measuring apparatus capable of removing drift components, random noises and the like in detection signals by simple processing.